fix(session): archive actual idle compact drops

nanobot/agent/memory.py

@@ -807,10 +807,9 @@ class Consolidator:
                 metadata={},
                 last_consolidated=0,
             )
-            probe.retain_recent_legal_suffix(max_suffix)
+            dropped, already_consolidated = probe.retain_recent_legal_suffix(max_suffix)
             kept = probe.messages
-            cut = len(tail) - len(kept)
-            archive_msgs = tail[:cut]
+            archive_msgs = dropped[already_consolidated:]
 
             if not archive_msgs and not kept:
                 session.updated_at = datetime.now()

tests/agent/test_auto_compact.py

@@ -76,10 +76,9 @@ def _make_fake_compact(
             metadata={},
             last_consolidated=0,
         )
-        probe.retain_recent_legal_suffix(max_suffix)
+        dropped, already_consolidated = probe.retain_recent_legal_suffix(max_suffix)
         kept = probe.messages
-        cut = len(tail) - len(kept)
-        archive_msgs = tail[:cut]
+        archive_msgs = dropped[already_consolidated:]
 
         if not archive_msgs and not kept:
             session.updated_at = datetime.now()

tests/agent/test_consolidator.py

@@ -440,6 +440,44 @@ class TestCompactIdleSession:
         assert "u0" not in user_content
         assert "u25" in user_content or "a25" in user_content
 
+    @pytest.mark.asyncio
+    async def test_non_contiguous_suffix_archives_actual_dropped_messages(
+        self,
+        real_consolidator,
+        mock_provider,
+    ):
+        """Assistant-only tails retain a non-contiguous slice, so archive the
+        actual dropped messages rather than a computed prefix."""
+        mock_provider.chat_with_retry.return_value = MagicMock(
+            content="Tail summary.", finish_reason="stop"
+        )
+        sessions = real_consolidator.sessions
+        session = sessions.get_or_create("cli:noncontiguous")
+        for i in range(15):
+            session.add_message("user", f"user-{i:02d}")
+        for i in range(10):
+            session.add_message("assistant", f"assistant-{i:02d}")
+        sessions.save(session)
+
+        result = await real_consolidator.compact_idle_session("cli:noncontiguous", max_suffix=6)
+        assert result == "Tail summary."
+
+        reloaded = sessions.get_or_create("cli:noncontiguous")
+        assert [m["content"] for m in reloaded.messages] == [
+            "user-14",
+            "assistant-00",
+            "assistant-01",
+            "assistant-02",
+            "assistant-03",
+            "assistant-04",
+        ]
+
+        archived_call = mock_provider.chat_with_retry.call_args
+        user_content = archived_call.kwargs["messages"][1]["content"]
+        assert "user-14" not in user_content
+        assert "assistant-00" not in user_content
+        assert "assistant-09" in user_content
+
     @pytest.mark.asyncio
     async def test_acquires_consolidation_lock(self, real_consolidator, mock_provider):
         """Verify lock is held during execution."""

tests/agent/test_session_manager_history.py

@@ -691,6 +691,6 @@ def test_retain_recent_legal_suffix_last_consolidated_correct_in_else_branch():
     # Retained messages start from latest user (u9) + max_messages forward
     # so retained = [u9, a0..a9][:4] → but these are from original indices 9..12
     # Of those, indices 9,10,11 are < 12 (before_lc), so new_lc = 3
-    assert session.last_consolidated <= 12
+    assert session.last_consolidated == 3
     # already_cons should count dropped messages with original index < 12
-    assert already_cons >= 0
+    assert already_cons == 9